1. Field of the Invention
The invention relates to methods and devices for treating fiber webs and in particular to methods and devices for preventing pressure peaks at the edges of a fiber web passing over a treatment roll having a pressing shoe.
2. Description of Related Technology
Pressing devices which include a pressing station comprising a roll and a counter roll defining a roll gap through which a material web is conveyed are well known. A pressing roll may include a pressing shoe disposed within a flexible shell that has a ring-like cross-section. The pressing shoe is disposed within the shell and presses against an inside surface thereof by utilizing pressing units and/or pressure relief units disposed over a width of the pressing shoe and distributed in a direction transverse to the direction of movement of a web through the roll gap.
If such a pressing device is utilized to remove water from a paper web, the two edges of the paper web may be dewatered more thoroughly than the other regions of the paper web, especially when the paper web being produced has a smaller width than that normally conveyed through the pressing device. In such a case, there must be a reduction in the pressing force in the area of the web edges.
Pressure relief units for press rolls disposed in various ways in the vicinity of paper web edges are known from J. M. Voith GmbH Company Document P 4007 (deposited in the Library of the German Patent Office (Office for Company Documents) on May 20, 1983). In such devices, the pressing shoe is disposed on a carrier body. A pressure chamber that can be filled with a pressure medium is provided between the pressing shoe and the carrier body. Pressure relief is provided by at least one ring element which extends in the direction of the pressing force. An interior space defined by the ring element is sealed against the rest of the pressure chamber and is connected to a pressure relief line. Preferably, in such a device, two ring elements are disposed next to one another at each end region of each shoe press.
To provide sufficiently sensitive pressure relief at the edges of fiber webs of various widths, the pressure in the outermost of the two ring elements must first be gradually decreased to a minimum pressure and then the pressure at the second (interior) ring element is decreased. Such a design may be disadvantageous because the control and adjustment of the ring elements is costly.